Dispersion of a layer silicate in thermoplastic resins is known to improve their mechanical, thermal, gas barrier or other properties. In the layered silicate constituting clay minerals, extremely fine, flaky crystals are held together by ionic bonding. The above-described properties of thermoplastic reins can be improved by disintegrating this aggregate structure by a chemical or physical means to thereby evenly disperse the flaky crystals throughout the thermoplastic resins.
For example, Japanese Patent Publication No. Hei 8-22946 discloses that a polyamide resin structure containing evenly-dispersed flakes of a layered silicate can be formed by intercalating aminocarboxylic acid into the layered silicate to initially enlarge spacings between adjacent layers, then inserting ε-caprolactam, which is a monomeric moiety of polyamide, into interlayer spaces and concurrently allowing it to undergo polycondensation.
However, in general, it is extremely difficult to achieve uniform dispersion of the layer silicate in a matrix of a polymer if it is disimilar to polyamide and its monomer can not be inserted into interlayer spaces of the layered silicate. Various attempts have been made to solve this problem.
For example, Japanese Patent Laying-Open No. Hei 9-183910 discloses a method for dispersing a layered silicate in a polymer by mixing, in a molten state, an organic dispersion incorporating an organically-modified layered silicate swelled and dispersed therein with a vinyl polymer compound. Japanese Patent Laying-Open No. Hei 10-182892 discloses that melt neading of an organically-modified layered silicate, a polyolefin oligomer containing a hydrogen-linkable functional group and a polyolefin polymer results in the preparation of a polyolefinic resin composite material in which a spacing between adjacent layers of the layered silicate is infinitely swelled in the polymer.
Meanwhile, resins have been conventionally used in the form of foams to reduce the weight or cost of the resins or to provide decorative appearances thereto. Incorporation of inorganic fillers in such foams has also been conventional to improve mechanical strength, heat insulation performance, impact absorption performance of the foams. For example, Japanese Patent Laying-Open No. Hei 8-143697 describes that incorporation of a layered silicate in a polypropylene foam composition results in the improved strength or other physical properties of such a foam.
However, the method described in Japanese Patent Laying-Open No. Hei 9-183910 requires the use of a solvent. The resulting composite material exhibits the insufficient strength, such as in flexural modulus, probably due to the insufficient removal of a residual solvent. Also, the inclusion of complex steps, such as of dissolving a polymer, swelling the organically-modified layered silicate and removing the solvent, makes this prior art impracticable from an industrial point of view.
Also, the material described in Japanese Patent Laying-Open No. Hei 10-182892 as containing the layered silicate in the form of crystalline flakes dispersed evenly in a polymer has been found extremely difficult for practical use as an industrial material.
That is, because a reaction between a functional group in the polyolefin oligomer and a hydroxyl group on a surface of the layered silicate is caused to occur during the melt kneading, the hydroxyl group of the layered silicate is not necessarily treated in an effective manner by the functional group. Accordingly, a large amount of polyolefin oligomer is required to achieve uniform dispersion of the layered silicate in practice. The high loading of such an oligomer component in the polymer is undesirable in terms of physical properties and cost.
Japanese Patent Laying-Open No. Hei 8-143697 discloses that a polypropylene foam composition if including a layered silicate with a blowing agent adsorbed therein provides a polypropylene foam with a high expansion ratio and high strength. However, disintegrating an aggregation structure of the layered silicate and evenly dispersing the resulting flaky crystals in the resin are left out of consideration. The disclosed technique thus fails to obtain a sufficient effect of the loaded layered silicate. Also, a specific blowing agent must be held adsorbed to the layered silicate. The requirement of a multi-stage treatment thus reduces productivity. Further, the use of a silane coupling agent is essential. This increases the cost. The unstable nature of the silane coupling agent which is highly linkable to a moisture in the air reduces handleability.
In view of the problems encountered with the above-described conventional thermoplastic foam compositions each containing a thermoplastic resin and a layered silicate and methods for production thereof, the present invention is directed to provide a thermoplastic foam which comprises a thermoplastic resin and a layered silicate and in which foam cells and layered silicate are evenly and finely dispersed, and also to provide a method for production thereof.